Broadband Nonlinear Absorbing Platinum 2,2′-Bipyridine Complex Bearing 2-(Benzothiazol-2′-yl)-9,9-diethyl-7-ethynylfluorene Ligands

Abstract

A platinum 2,2 0 -bipyridine complex (1) bearing 2-(benzothiazol-2 0 -yl)-9,9-diethyl-7-ethynylfluorene ligands was synthesized and characterized. Its photophysical properties and nonlinear absorption characteristics were systematically investigated by UV-vis absorption, emission, and transient difference absorption spectroscopy, as well as Z-scan and nonlinear transmission techniques. Complex 1 exhibits a strong structureless 1 π,π* absorption band at 374 nm and a broad, weak metalto-ligand charge transfer ( 1 MLCT) transition in the visible region in CH 2 Cl 2 solution. It emits at approximately 565 nm with vibronic structures at room temperature in polar solvents, attributed to the acetylide ligand 3 π,π* excited state. In low-polarity solvents such as hexane and toluene, the emission band becomes structureless and red-shifted, which is assigned to the 3 MLCT state. The emission spectrum becomes more structured and slightly blue-shifted at 77 K in butyronitrile glassy matrix. In femtosecond and nanosecond transient absorption measurements, 1 exhibits both singlet and triplet excited-state absorption from 450 to 800 nm, which are tentatively attributed to the 1 π,π*/ 1 MLCT and 3 π,π*/ 3 MLCT, respectively. Z scan experiments were carried out using nanosecond and picosecond pulses at 532 nm, and picosecond pulses at a variety of other wavelengths in the visible and near-IR, and the experimental data were fitted by a five-level model using the excited-state lifetimes and estimated cross-section values from the photophysical study. In this way, values were obtained for the first and second singlet excited-state absorption cross sections and the triplet excitedstate absorption cross section throughout the visible and near-IR and for the two-photon absorption (TPA) cross section in the near-IR region. Our results demonstrate that 1 possesses extremely large ratios of the excited-state absorption cross sections to the ground-state absorption in the visible spectral region and, compared to the other two-photon absorbing platinum complexes, the largest two-photon absorption cross sections in the near-IR region. This makes complex 1 a very promising candidate for photonic devices that require large and broadband nonlinear absorption. Reverse saturable absorption of 1 in CH 2 Cl 2 solution at 532 nm for a nanosecond laser pulse was demonstrated. A remarkable transmission decrease was observed when the incident fluence increased.

Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

“…Instead, it resembles the spectral feature of the 1 π,π*/ 1 MLCT absorption from the bipyridyl Pt II benzothiazolylfluorenylacetylide complex. [14] Therefore, the observed fs TA spectrum of 1 is tentatively attributed to the acetylide ligand-localized 1 π,π* state mixed with 1 MLCT/ 1 LLCT states. For complex 2, the peaks of its TA spectrum line up well with those of the NDI radical anion and the radical anion of the complex at ca.…”

“…This assignment is supported by the similar absorption band maximum of the TA spectrum of 1 to that of the acetylide ligand and the broadband feature similar to that of the Pt II bipyridyl acetylide complexes with the same acetylide ligand. [14] In contrast, the TA spectra of 2-4 are quite similar to that of the 3 NDI -· radical anion. [42,43] However, the lifetimes are much shorter than that of the pure 3 …”

“…The slightly structured feature of the emission in CH 3 CN implies more CT states is consistent with that reported for other similar Pt II diimine acetylide complexes. [14,41] In contrast, the emission of complex 4 in different solvents exhibits a minor solvatochromic effect. Table 5 summarizes the emission wavelengths, lifetimes, and quantum yields for complexes 1 and 4 in different solvents.…”

“…[18,27] In addition to the aforementioned work, Weinstein's group reported a detailed study on the excited state dynamics of a Pt II diimine chloride complex bearing a strong electron-withdrawing naphthaldiimide (NDI) substituent on the diimine ligand. [39] It was found that there is little electronic interaction between the Pt(phen)Cl 2 [14,18,20,23,24,27] In this work, we synthesized a Pt II diimine complex and two cationic Ir III complexes bearing the NDI motif on the diimine (N ∧ N) ligand (complexes 2-4 in Figure 1). Pt II complex 1, which contains the 1,10-phenanthroline ligand, was synthesized to be a reference compound for the Pt II complex 2 in order to better understand the role of NDI in the presence of the BTF acetylide ligands.…”

“…promising candidates for applications in organic light-emitting devices (OLEDs), [3][4][5][6] low-power upconversion, [7][8][9] luminescent bioimaging, [10][11][12][13] nonlinear optical devices, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] etc. In addition, the chemical structures of Pt II and Ir III complexes are readily modified to tune the photophysical properties in order to tailor the specific requirements for each application.…”